Vee hull features.

[ozzie-crawl]

i actually started a thread acouple of days ago on this same thing but with cats as i havent been able to find an answer.
sorry simon not trying to jack your thread

[paulwilliams]

the further out the prop the less tendency to porpoise but more chance of chine walk, i think this is were the 10% comes in as it is a compromise between the 2

That says it all

There are no "hard and fast" rules, just a set of generalisations that can be used as a "first guess", unless, of course, you're building a proven design and have an explicit set of setup instructions from the designer regarding balance point, prop location, rudder mounting point etc.

As you're building your own design, then you will have a certain amount of tinkering to do in order to get the best from your boat - you are very unlikely to hit the exact setup first time.

Bear in mind that altering one thing to eliminate one problem can quite easily cause another. For example, the further out you place the prop from the hull's CofG, the more leverage it will have and the more prop walk you are likely to get. I say "likely" because it's unknown - you may, or may not, it's merely theoretical.

Another example is prop depth. It is usually the case that raising the prop reduces motor load, which is good, but it also usually increases prop torque effects, ie., prop walk and torque roll, which is bad.

Paul

[ozzie-crawl]

the further out the prop the less tendency to porpoise but more chance of chine walk, i think this is were the 10% comes in as it is a compromise between the 2

[Simon.O.]

O.k I now have enough info on strut height and angle.
Next is :
Prop distance from transom
I understand that 10 % of hull length is normal.
I can see the problems that may arise if it is less

What happens when it is out further ?
Faster ?
Less prone to blow over ?
Poor steering ?

What else ??

[crrcboatz]

The monos that have "hook"built into them are few. The designers had the experience to know what they are doing with the hook design in the first place. I would apply the KISS rule, aka, keep it simple ______. The 2 that come to mind do not use trim tabs and the hook is as someone earlier mentioned, near the transom.
Do yourself a favor and get that hook out of the boat you are building. Someone like you with the God given ability to build from scratch, I covet your skills, should not put themselves through the trial and error of what a hooked bottom can cause you in the way of heartache. Man I wish I had the skills to do what you are doing!!!!!

Curt

[785boats]

It's true that raising the angle of the strut too much causes the hull to run very loose, & dropping the strut too much causes the hull to run too wet. Everything has a point of 'diminishing return'. It's a question of finding the right balance with the strut angle, the COG & the type of prop being used to get that slight nose up attitude of the hull in full flight. Again we are only talking about 1 degree or so.
Simon.
If you are using a flex drive, with either a strut setup or stinger type setup, I believe as low as you can get it is best for the exit.
As an example the centerline of the stinger on mine is 8mm above the bottom of the keel.

Paulwilliams.
I too have just been reading through your website. Great stuff. Going back there now for some more learnin'
Cheers.
Paul.

[paulwilliams]

I have just been looking at your website Paul there is a whole lot of very good information
Adrian

Thanks!

[DISAR]

Very nice info here about monos. I just want to add that I was told in the past that for a Delta Force hull the centre of the shaft should be about 12mm - 0.47" above the keel, and when placing the strut too deep creates lift on the transom and not proper handling. Unfortunately I have not tried different strut heights to know the difference.

[EPower]

In my experience, if you raise the prop too far out of the water the boat becomes very unstable, and will be more likely to spin out or flip. Raising the strut allows you to use bigger props and unloads the motor, but at the (probable) expense of stability, thanks to increased torque effects.

Running the prop deeper loads the motor more, but does calm things down a lot.

However, these are generalisations - until you build the thing and run it, you simply won't know.

As Graham says, typically you run the shaft as low in the apex of the vee as possible, unless you have a European-style shallow vee mono with a solid shaft - these often exit the transom quite high up.

Paul

I have just been looking at your website Paul there is a whole lot of very good information
Adrian

[paulwilliams]

In my experience, if you raise the prop too far out of the water the boat becomes very unstable, and will be more likely to spin out or flip. Raising the strut allows you to use bigger props and unloads the motor, but at the (probable) expense of stability, thanks to increased torque effects.

Running the prop deeper loads the motor more, but does calm things down a lot.

However, these are generalisations - until you build the thing and run it, you simply won't know.

As Graham says, typically you run the shaft as low in the apex of the vee as possible, unless you have a European-style shallow vee mono with a solid shaft - these often exit the transom quite high up.

Paul

[G Doggett]

Hi Simon

The strut height on my DV hulls has always been determined by how low I could mount the rear of the stuffing tube inside the rear of the hull. Simple as that !
In most cases the shaft then extends parallel with the keel, with the prop distanced behind the transom at 10% of the hull length.
For example a 30 inch hull has prop at 3 inches behind transom.
I think that if you have to angle a strut more than 1 - 2 degrees up or down , you are compensating for a problem somewhere else.
Works for me.
Graham.

[Simon.O.]

Paul, that too has been printed and filed in my reference folder.
Strut angle is now sorted. One down one to go.

Let's see who can give me the beans on strut height for monos.

[785boats]

Strut height.
I am happy with what strut angle does on a mono, and have a rough idea on what strut height can do.
Raising the strut will unload the prop increasing rpm but at the same time lower the transom wetting the rear and inducing drag. These are two opposing forces.
Have I got this bit right or is my theory off here.

Simon
Nearly.
By raising the angle of the prop, yes the transom is pushed down into the water slightly but this increases the angle of attack of the hull. This allows three things to happen.
Less hull in the water, just the last couple of inches or less, instead of 10 or 12 inches if the hull is running flat or "Wet" The amount of friction on the water that this reduces gives a marked reduction in load on the motor & therefore temperatures, while resulting in higher revs & speed.
The angle of attack of that part of the hull in contact with the water causes the hull to ride up higher on top of the water also.
More air also gets under the hull to help lift it because of the angle of attack. I know you are familiar with this concept with your work on riggers & hydros.

The cumulative effect of the above is less drag, increased speed & reduced load on the electrics. This far outweighs the slight downward pressure created by the increased angle of the thrust line. After all we are only talking of about 1 or 1-1/2 or 2 degrees at the most.

Maybe someone else with more experience than me can chime in & add more info regarding the finer points of strut angles.
Hope this helps a bit.
Paul.

[Simon.O.]

Paul and Paul. Thanks for the very detailed answers.
As I am building this I can do as I please, my boat used for fun and straight runs.

I will finish it flat in regards to the bottom and rocker/hook.
I have found that it will need 2 more lengthwise rails inside the hull skin from amidships to the transom. This will flatten the hook and prevent any flexing. Sealed and varnished 3mm balsa on cross grain doesn't flex a lot anyway. I will just be making it a little stiffer.

I hear you both on the trim tab, spray rail and torque wedge bit and will keep all of these featurse off unless they are needed. Any laterral ballast can be induced by moving the rx and pack to the left, any more an I can either add hull features or weight.
Moving on.

Strut height.
I am happy with what strut angle does on a mono, and have a rough idea on what strut height can do.
Raising the strut will unload the prop increasing rpm but at the same time lower the transom wetting the rear and inducing drag. These are two opposing forces.
Have I got this bit right or is my theory off here.

Given a 30" hull with 22deg deadrise roughly where should the shaft centre line be for kick-off. I use the shaft CL as a measure as there are so many different struts that the CL is a more accurate measure.
I will be running a 062 wire down a flooded shaft box so I will have the ability to adjust up to 5/16" or 8mm vertical movement on the strut.
440 prop for starters.

What say the clever people ??

[785boats]

Hi Simon.
I run one of those 25"Delta Force copies & that hull has a hook but is only about 4inches long. Very stable without trim tabs.
The way I look at it is this.
The hook as Paulwilliams suggested tends to push the nose down.
By angling the strut up slightly the driveline thrust raises the nose pivoting on the transom. This balances out the force of the hook.
This then allows the hull to ride up on the trailing edge of the hook (transom) & run a lot drier. When the sweet spot for the COG is found by moving the batteries the hull should run with a slight nose up attitude & just on the transom & prop.
If the nose tends to keep lifting & wants to flip drop the strut angle a tad and/or move the packs forward a bit.
If it looks too wet do the opposite.
This is purely subjective on my part, but I consider my hull well trimmed if it leaves the water after hitting a bit of chop or another boats wake & flies through the air at exactly the same attitude as on the water & lands the same way. In my opinion, the thrust line, the COG, the attitude & the aeronautics are all in perfect harmony. Like I said, purely subjective & based on nothing except that it looks right.
Also being able to do 5 laps at full noise around our course without lifting off & under control most of the time means that it's about right too.
All the best in your new designs.
Paul.